1. Field of the Invention
The present invention relates to a method of manufacturing a nonvolatile memory device.
2. Description of the Related Art
In recent years, a resistive random access memory (ReRAM) that stores, in a nonvolatile manner, resistance information, for example, a high resistance state and a low resistance state of an electrically rewritable variable resistive element attracts attention as a nonvolatile memory device. In such a ReRAM, for example, variable resistance memory cells in which variable resistive elements as storage elements and rectifying elements such as diodes are connected in series are arranged in an array shape in intersections of a plurality of bit lines extending in parallel to a first direction and a plurality of word lines extending in parallel to a second direction perpendicular to the first direction (see, for example, Myoung-Jae Lee et al., “2-stack 1D-1R Cross-point Structure with Oxide Diodes as Switch Elements for High Density Resistance RAM Applications”, IEEE, pp. 771-774, 2007). Examples of the variable resistive elements include metal oxides such as NiO, a high resistance state and a low resistance state of which can be switched according to control of a voltage value and voltage application time.
Such the ReRAM can be manufactured by a method same as a method of manufacturing a field-programmable ROM that has columnar structure in which diode layer and insulating layer are connected in series in cross-points of a plurality of first wires extending in parallel to a first direction and a plurality of second wires extending in parallel to a second direction perpendicular to the first direction (see, for example, S. B. Herner et al., “Vertical p-i-n polysilicon diode with antifuse for stackable field-programmable ROM”, Electron Device Letters, IEEE, vol. 25, no. 5, pp. 271-273, May 2004). For example, first, a diode material as a material of the rectifying element, a variable resistive material as a material of the variable resistive element, and a metal material as a material of an electrode are deposited in order. Subsequently, a resist applied on the metal material is exposed and developed by the photolithography technique into a desired pattern to form a mask. Thereafter, the metal material, the variable resistive material, and the diode material are etched by the anisotropic etching using the mask. Consequently, the ReRAM can be formed.
However, because various deposited films are collectively processed in such a process, a mask material having thickness sufficient for withstanding the processing is necessary. A processing technique with a high ratio of depth to width of a groove to be etched (hereinafter, “aspect ratio”) is required. The likelihood of collapse of patterns of variable resistance memory cells formed in a columnar structure increases after the processing because of the high aspect ratio.